The present invention relates to a motor control apparatus, more particularly, to an improved motor control apparatus having a thyristor chopper.
In recent years, circuits to control electric power supplied to electric motors have been proposed, in which the voltage is applied to the motor intermittently at a predetermined period through a chopper circuit using thyristors or power transistor so that the average value thereof is maintained at a desired value. In such a control circuit, it is desired, in addition to reduction of size, that a duty factor of the chopper circuit, i.e., the ratio of conductive time to the sum of conductive and non-conductive times be controlled over a wide range.
In the article by R. E. Morgan "Time Ratio Control with Combined SCR and SR Commutation", IEEE Trans. on Communications and Electronics, Vol. 83, pages 366-371, July, 1964, there is shown a commutating circuit for TRC that combines an auxiliary SCR and a SR. This commutating circuit contributes to the reduction of the cost and size of the auxiliary SCR. However, it has drawbacks in that two kinds of reactors L.sub.C and SR.sub.C are necessary, the former having a relatively high inductance and the latter being a saturable reactor, and in that commutating current through the auxiliary SCR is always determined by commutating elements of a capacitor C.sub.C and the reactor L.sub.C.
Other relevant prior art is as follows:
(1) W. McMuarry, "Silicon-Controlled Rectifier D-C to D-C Power Converters", IEEE Trans. on Communication and Electronics, Vol. 83, pp. 198-203, March, 1964. PA1 (2) U.S. Pat. No. 3,875,486, "Motor Speed Control Circuit". PA1 (3) U.S. Pat. No. 3,903,465, "Chopper Control System".
This shows various ways in which saturable transformers are used in silicon-controlled rectifiers.
This shows Jone's chopper circuit having a transformer. This Jone's chopper, however, differs from an ordinary chopper circuit in construction and operation.
In this chopper control system, the application of the chopper-off signal to the chopper is prohibited by a prohibit circuit.